In offset printing, printed images are transferred from printing plates mounted on plate cylinders to a moving web of material by transfer cylinders known as blanket cylinders. Typically, printing blankets are mounted on the blanket cylinders which have a rubber surface for transferring the printed images. Conventional printing blankets are rectangular in shape and mounted in axial gaps extending across the circumferential surface of the blanket cylinders. One problem with this design is that at high operational speeds the gap in the blanket cylinder causes vibrations in the printing press which have the effect of varying the optical densities of the printed image. There are a number of other problems associated with this design which affects the quality of the final printed product.
Many of these problems have been solved by making the printing blankets gapless and tubular in shape. However, this improved design has created several problems of its own.
To mount the tubular printing blankets, an air canal is provided at one end of the blanket cylinder. The canal supplies pressurized air radially outward through a plurality of passages. As a printing blanket is placed over the passages, the pressure from the exiting air radially expands the printing blanket enabling it to be axially mounted onto the circumferential surface of the blanket cylinder. Since the inner circumference of the printing blanket is slightly smaller than the outer circumference of the blanket cylinder, once the printing blanket is mounted it is stressed in tension by the blanket cylinder to provide a tight pressure relationship between the printing blanket and the blanket cylinder. This pressure relationship fixes the printing blanket on the blanket cylinder so that there is no relative movement therebetween during operation of the press.
A problem with this arrangement is that air gets trapped at the interface of the printing blanket and the blanket cylinder. During operation of the press this trapped air creates a continually advancing wave in front of a nip between the blanket cylinder and the cylinder against which it is pressed causing the printing blanket to bulge. This phenomena is known as printing blanket procession. It creates defects in the printed product by forming latent double images.
Several attempts have been made to reduce or eliminate blanket procession, but none have been successful. One attempted solution was to increase interference between the printing blanket and the blanket cylinder. Another was to change the material combination of the printing blanket and the blanket cylinder surface to a combination having higher coefficients of friction. Both of these attempted solutions failed, since the primary cause of procession does not involve slippage of the printing blanket relative to the blanket cylinder.
Another attempted solution was to decrease the normal forces between the blanket cylinder and its adjacent cylinders, i.e., the printing cylinder and impression cylinder or other blanket cylinder. Although this solution reduces the rate of procession, it also reduces the quality of print to an unacceptable level. Still another solution was attempted which involved mechanically fixing the printing blanket to the blanket cylinder. This attempt was also unsuccessful because the printing blanket was too thin to withstand the forces required to stop the procession, and hence the printing blanket would tear.
The deficiencies in each of these attempts are fundamental and cannot be eliminated.